With Congress debating a major expansion in the program for guest workers, the nation’s largest union of farmworkers planned to announce today that it had signed the first nationwide contract covering agricultural guest workers.

The union, the United Farm Workers, and Global Horizons, a labor contractor based in Los Angeles, have signed an agreement that provides employer-paid medical care, a seniority system and a grievance procedure to help ensure that farms comply with state and federal laws.

“This is huge, because we’re setting a whole new standard of rights and benefits for guest workers,” said Erik Nicholson, the union’s Pacific Northwest director. “Guest workers have been the most exploitable and vulnerable workers short of slaves in the United States.”

Global Horizons, one of the nation’s largest suppliers of agricultural guest workers, has nearly 1,000 in the country now, but plans to have 3,000 to 5,000 by peak season this summer.

The company, which has workers in more than a dozen states, signed the union contract in part to help improve its image after Washington state revoked its license to do business there because of violations alleged by state investigators. The state accused it of failing to pay Thai guest workers their promised wages, putting them in inadequate housing, not paying enough unemployment insurance taxes and improperly withholding state income taxes.

Gov. Christine Gregoire of Washington revoked Global’s license in December, even though the company had agreed three months earlier to a $230,000 settlement, which was used largely for workers’ unreimbursed airfare and for improperly deducted taxes. Global Horizons said it had committed only minor violations.

Mordechai Orian, the president of Global Horizons, said the union contract would help defuse complaints that guest worker programs treated workers poorly. At a time when many fruit and vegetable growers are having a hard time finding enough workers, Orian said the contract would also help persuade guest workers to stay in agriculture instead of leaving soon after arriving in the United States to work in other industries.

“It’s definitely going to cost us more money,” Orian said, asserting that the guest program was a good one for workers, and would soon be better as a result of this contract.

Agricultural guest workers, who enter the country under the H-2A program, often remain for six months to two years. Under federal law, labor contractors and growers can import such workers only after showing that there are not enough workers in the nation to take the farm jobs.

“From the bracero program of the 1940s, ’50s and 60s to the present-day H-2A program, the use of agricultural guest workers has been beset by violations of legal protections,” said Arturo Rodriguez, president of the United Farm Workers. “For the first time, a union contract will protect agricultural guest workers from retaliation for complaining about conditions or treatment.”

In 2004, another farmworkers’ union, the Farm Labor Organizing Committee, signed a contract that protects several thousand guest workers in just one industry in one state, cucumber workers in North Carolina.

Under the contract with Global Horizons, the guest workers will be paid 2 percent more than the minimum wage that guest workers normally receive. In Washington state, for instance, the federal minimum for agricultural guest workers is $9.01.

The contract also includes three-day bereavement leave as well as round-trip transportation if a family member dies back home.